Electrode grade graphite is manufactured from a commercial grade of coke having an acicular, anisotropic microstructure called needle coke, see U.S. Pat. No. 2,775,549 to Shea, Dec. 25, 1956, Cl. 201-42, made by delayed coking of certain petroleum residues under specific conditions of heat and pressure. To produce graphite from such coke, it is necessary to heat it to a temperature in the range of 2000.degree.-3000.degree. C., which has the dual function of supplying energy for the conversion of the carbon in the coke to the graphitic crystalline form and of volatilizing impurities. When carbon bodies made from such cokes are heated at temperatures in the vicinity of 1000.degree.-2000.degree. C., various sulfur-containing compounds decompose, attended by a rapid and irreversible expansion of the carbon body. This phenomenon is termed "puffing". During the production of graphite articles, particularly high performance graphite electrodes, puffing is extremely undesirable as it may destroy the structural integrity of the piece and render it marginal or useless for its intended purpose.
Puffing of a carbon article made from high sulfur cokes generally starts at about 1500.degree. C., and may result in a volumetric expansion of as much as 25%. It is not simply an elastic expansion but should be characterized as an inelastic, irreversible expansion.
The generally accepted explanation of the puffing phenomenon is that in acicular needle cokes with a relatively large amount of sulfur, sulfur atoms are bonded to carbon atoms by covalent bonds, either in carbon ring structures or linking rings. These bonds are less stable at high temperatures than the carbon-to-carbon bonds. On heating, the carbon-sulfur bonds rupture, the sulfur is freed, then reacts with hydrogen to form hydrogen sulfide. The simultaneous rupture of these bonds and evolution of hydrogen sulfide and other sulfur containing materials causes the physical expansion called puffing.
Puffing has been avoided in the past by using coke made from petroleum residues low in sulfur content. This approach is of only limited utility at present since the principal petroleum crudes currently in use have high sulfur contents, and the cokes made from their residues such as decant oil from catalytic cracking or thermal tar, will normally exhibit an undesirable degree of puffing.
Another approach to elimination or alleviation of the puffing problem in manufacture of graphite articles has been by the use of additives. These additives have usually been added during the mixing stage when various sizes and grades of coke particles are mixed, before being wetted with pitch binder, formed into the desired shape, baked at an intermediate temperature and graphitized at high temperatures. Additives have included primarily metal salts and oxides, as disclosed in British Pat. No. 733,073, Greenhalgh, July 6, 1955, Cl. 90 b; French Pat. No. 1,491,497, Gillot et al., Aug. 11, 1967, Cl. C 01 b; French Pat. No. 2,035,273, Continental Oil, Dec. 18, 1970, Cl. C 10 b 57; U.S. Pat. No. 3,642,962, Wallouch, Feb. 15, 1972, Cl. 201-17; U.S. Pat. No. 3,563,705, Grindstaff et al., Feb. 16, 1971, C. C 01 b 31/04, Cl. 423-375; U.S. Pat. No. 3,842,165, Grindstaff et al. Oct. 15, 1974, Cl. C 01 b 31/04, Cl. 264-29.1; and U.S. Pat. No. 3,338,993, Juel et al. Aug. 29, 1967, Cl. 106-56.
The patents above disclose the use of iron, sodium, chromium, nickel, cobalt, boron, aluminum, titanium, calcium, zirconium, manganese, magnesium, barium and strontium compounds as puffing inhibitors. Some compounds of this group are in general usage and of these a choice is naturally made based upon the effectiveness as a puffing inhibitor and upon other properties of the graphite article such as electrical resistivity, tensile strength, modulus of rupture, modulus of elasticity, coefficient of thermal expansion, and cost.
Of the above, French Pat. No. 1,491,497 discloses the use of chromium oxide at 0.2-5% in a mixture with coke and a binder as a catalyst, enabling graphitization to occur at temperatures in the range of 1200.degree.-2000.degree. C.
French 2,035,273 discloses a low sulfur coke produced by the addition of 0.3-5% of sodium carbonate to the coking stream mixture and subsequent hydrogenation of the coke at high temperature.
British 733,073 discloses the use of oxides of chromium, iron, copper, or nickel incorporated in the grinding stage of coke, mixed with pitch, shaped, baked at 1200.degree. C., and graphitized at 2500.degree.-2800.degree. C.
U.S. Pat. No. 3,563,705 discloses the use of mixtures of iron or calcium compounds with small amounts of titanium or zirconium compounds as puffing inhibitors incorporated into the coke-binder mixture.
U.S. Pat. No. 3,338,993 discloses the use of calcium, magnesium, strontium, and barium fluorides as puffing inhibitors with raw or calcined coke and binder, mixed, shaped, baked and graphitized.
U.S. Pat. No. 3,642,962 discloses the use of 1-3% calcium cyanamid or calcium carbide as desulfurizing agents and puffing inhibitors, mixed with raw coke prior to calcining.
U.S. Pat. No. 3,873,427, Long, issued Mar. 25, 1975, Cl. 201/17, discloses the addition of metallic chloride and ferruginous material for desulfurization of coke.
U.S. Pat. No. 4,043,898, Kegler, issued Aug. 23, 1977, Cl. 208/50, discloses delayed coking of selected feedstocks to produce needle cokes.
At present, the most common methods of the above are those using iron oxides mixed dry in the coke-pitch binder blend as puffing inhibitors. These are effective puffing inhibitors but must be used with caution, as their use tends to increase the coefficient of thermal expansion or CTE, of the finished product, to an undesirable level.
The coefficient of thermal expansion (CTE), which is conventionally expressed in in./in./.degree.C. or cm./cm./.degree.C., is also of vital importance in the production of graphite for certain applications. Electrodes for electric furnace melting of steel must have a low CTE to avoid excessive differential expansion at operating temperatures and the resultant spalling, which in turn causes excessive consumption of the electrode in operation. Other applications requiring dimensional stability at high temperatures are well-known although of somewhat less economic importance.
In general, the addition of any foreign material to a graphitizing carbonaceous mix will have, in addition to its desired effect, such as puffing inhibition, the effect of increasing the CTE of the graphite body.
A needle coke is distinguished by its physical structure when microscopically examined, showing long needle-like acicular particles. Such cokes, to be suitable for manufacture of graphite electrodes to be used in ultra-high powered electric steel furnaces, should have a graphite CTE characteristic of less than 5+10.sup.-7 /.degree.C. measured over the range of 0.degree.-50.degree. C. Needle cokes for lower powered electric steel furnaces may have a graphite CTE characteristic of as much as 7.times.10.sup.-7 /.degree.C. over the 0.degree.-50.degree. C. range.
The cokes or blends of cokes must be thoroughly mixed with the puffing inhibitor to avoid the difficulties present in making uniform homogeneous blends and to thoroughly coat the particles, which are often as much as 7 mm. in diameter. Both of these difficulties can lead to non-uniform dispersion of the inhibitor and to puffing, even though there is sufficient inhibitor present in the total mix to prevent puffing. This non-uniformity is particularly troublesome when operating under the newer type of graphitization processes, which raise the temperature of the carbon bodies (i.e. electrodes) at a much higher rate than the older processes. The combination of high sulfur with high rate of temperature rise exacerbates the problem and requires undesirably slow heating rates to overcome puffing.
It should be emphasized that overcoming the puffing problem becomes increasingly more difficult in the larger graphite electrode sizes (above 20 in. ((51 cm.)) diam) because in such sizes, larger particles of coke are used. Since the puffing inhibitor only coats the surface of the particles, the coke surface area to inhibitor weight ratio decreases, for a given weight addition ratio, giving a higher concentration of inhibitor on the coke particle surfaces for the larger particle blends. Thus a large amount of the inhibitor is at relatively greater distance from the centers of the coke particles in the larger coke particle mixes as opposed to the smaller particle mixes used in smaller electrodes. Migration of the inhibitor into the centers of the large particles becomes progressively more difficult and less effective as the coke particles increase in size.
The puffing problem is further increased with the rate of graphitization of the carbon bodies. Optimum distribution of the inhibitor throughout the structure of the carbon body to be graphitized is essential as the degree of puffing for any coke particle blend is highly rate sensitive, being directly related to the rate of temperature increase during the graphitization cycle. Thus, the figures in certain of the examples given will show a much higher dynamic puffing at a 14.degree. C./min. temperature rise than for a 5.degree. C./min. rise.
The amount of puffing for any given coke-inhibitor blend could be expressed as a proportionality in the general form:
______________________________________ ##STR1## where S = sulfur content of coke P = mean particle size .DELTA.T = rate of temperature where DP = dynamic puffing increase 1 = amount of inhibitor K = proportionality factor ______________________________________
Thus it may be seen that increases in sulfur content, particle size, and temperature rise will increase puffing, while an increase in the inhibitor level will decrease puffing.